Pressure-sensitive adhesive compositions generally will function, under normal circumstances, against the substrate it was designed to be adhered to. However, conventional adhesives do not function adequately to that substrate when contaminated with grease or oil. The problem of adhering to oil contaminated surfaces with conventional pressure-sensitive adhesives, as a general problem, is recognized in the art. For example, U.S. Pat. No. 3,991,002 describes a method for improving adhesion of normal pressure-sensitive adhesive tapes to oily or greasy substrates by treating the oil-contaminated substrate with a primer. This primer comprises a rubbery phase of an A-B-A triblock copolymer, such as a styrene-butadiene or styrene-isoprene block copolymer, and a resin blend of a resin compatible with the conjugated diene portion of the block copolymer and a resin compatible with the monovinyl aromatic hydrocarbon portion of the block copolymer (i.e., styrene). These components are placed in a solution, then applied as an aerosol to render the oil-contaminated surface stable for subsequent adhesion by a pressure-sensitive adhesive. This patent states that the conventional approach to adhering to oil-contaminated surfaces is a complicated and time-consuming clean-up prior to being adhered to by a conventional pressure-sensitive adhesive. However, the primer solution to the problem of oil contaminated surfaces proposed in this patent is still somewhat time consuming and impractical for most consumer applications.
U.S. Pat. No. 4,080,348 describes a pressure-sensitive adhesive composition which is stated to have superior "finger tack" as well as the ability to adhere to oily surfaces, such as skin. The adhesive disclosed contains a high proportion of simple diblock styrene-isoprene block copolymer admixed with a minor portion of a triblock styrene-isoprene-styrene or styrene-butadiene-styrene linear block copolymer tackified with solid tackifying resins. The specific formulations exemplified were alleged to have superior finger tack and the ability to adhere to skin.
Generically, the art has recognized adhesive compositions based on block copolymers of polyvinyl arenes blocks and polydiene blocks, as per the U.S. Pat. No. 4,080,348 above. However, most of the block copolymer-based adhesive compositions specifically taught in the art are based on polyvinyl arene blocks of polystyrene and polydiene blocks of either polybutadiene or polyisoprene or hydrogenated versions thereof. Continued work in the field has identified specific adhesive formulation based on these block copolymers as having particular properties useful in a variety of particular end uses. For example, U.S. Pat. No. 3,935,338 describes typical elastomeric structures having polyisoprene and polybutadiene elastomeric blocks in various configurations used in adhesive compositions for high temperature applications. The adhesive in this patent is hot-melt applied. The adhesive is tackified with a tackifying resin compatible with the diene elastomeric block B, and a second reinforcing resin compatible with the monoalkenyl arene block, which second resin has a higher softening point than the first resin. It is stated that this formulation provides an adhesive that has superior high-temperature performance as a pressure-sensitive adhesive. The specific elastomeric block compatible tackifying resin exemplified in this patent has a softening point of about 95.degree. C. Higher softening point resins were not characterized as being compatible with the elastomeric midblock. The limited teaching in this patent is exemplary of an underlying problem in the art. Resins having higher softening points have generally been found to be incompatible with the isoprene block copolymer elastomeric midblock, even when lower molecular weight versions of the same resin do not display this compatibility problem. Generally, adhesives tackified solely with these high softening point resins exhibit unacceptable aging and high temperature adhesive performance. The ability to use these higher softening point resins, that otherwise chemically tend to be midblock compatible resins, would be extremely desirable in terms of obtaining stable high temperature adhesives.
An alternative approach to heat stability is proposed in U.S. Pat. No. 4,028,292, which uses a heat-stabilizing additive to improve the high-temperature performance of a conventional block copolymer-based pressure-sensitive adhesive. This patent notes that the use of lower softening point materials to get acceptable pressure-sensitive adhesive processing and performance at lower temperatures leads to the adhesive having unacceptable strength in the temperature range of 100.degree.-150.degree. F.
U.S. Pat. No. 4,652,491 discusses an adhesive designed to firmly adhere to polyolefin surfaces. The adhesive composition described is a blend of a block copolymer of polystyrene and polybutadiene blocks, with styrene-butadiene random copolymer or natural rubber, admixed with a lower softening point tackifier. It is noted in this patent that certain higher softening point tackifiers are ineffective in providing an adequate adhesive when used on their own, however, these tackifiers allegedly may be used in minor amounts when employed with effective amounts of lower softening point, elastomeric block compatible, tackifiers.
U.S. Pat. No. 4,792,584 describes an adhesive composition based on a block copolymer of polystyrene and polybutadiene blocks, having a specific ratio of 1,2 butadiene content to vinyl aromatic content. It is also disclosed in this patent that creep resistance of the adhesive can be improved by the use of reinforcing resins compatible with the styrene-block of the copolymer. No oil-contamination tolerance abilities or heat aging stability are attributed to these adhesive compositions.
U.S. Pat. No. 4,810,574 describes a diaper tape adhesive formulated from a blend of a block copolymer of polystyrene and polybutadiene with about 30-50% of a petroleum hydrocarbon resin having a piperylene component, with a chain transfer component, and a vinyl substituted styrene having a softening point from 0.degree.-40.degree. C. This adhesive is described as providing a tape that will repositionably, rather than firmly, adhere to a polyolefin material, particularly a diaper polyethylene cover sheet. The patent attributes no oil-contamination tolerance, heat stability or age stability to its adhesives. Generally the art fails to identify the above noted problems or suggest a solution thereto.